This invention relates generally to a staged catalytic process for reducing the ammonia concentration in the gas produced by a gasifier, particularly an oxygen-blown coal gasifier in an integrated gasification combined cycle power plant equipped with high temperature desulfurization.
In integrated gasification combined cycle (IGCC) power plants, low Btu fuel gas produced by a gasifier is burned and expanded through a gas turbine, and the exhaust heat from the gas turbine is used to generate steam for a steam turbine. The low Btu fuel gas can be produced by gasifying coal, biomass, municipal solid waste, wood chips, heavy residual oil, petroleum coke, refinery wastes and other materials. As used herein, the term "fuel gas" refers to gas produced by any such gasification process. IGCC systems are attractive because of their high efficiency and because they can use relatively abundant and/or inexpensive energy sources.
Since the fuel gas produced by gasification typically contains high levels of hydrogen sulfide (H.sub.2 S), a sulfur removal system must be employed. Currently, both low temperature and high temperature desulfurization schemes are used. Hot gas clean up (HGCU) is a high temperature sulfur removal scheme which has several advantages over low temperature schemes, most notably increased system efficiency and decreased cost. HGCU reduces the sulfur in the fuel gas to less than 50 ppmv H.sub.2 S and is typically carried out in the range of approximately 800-1200.degree. F. This temperature regime is near optimal for desulfurization because at temperatures below about 800.degree. F. the overall power plant efficiency decreases, while at temperatures above about 1200.degree. F. the efficiency and stability of the desulfurization sorbents decrease. However, high temperature fuel gas tends to have a high ammonia content, about 1000-2000 ppmv. This high ammonia content results in high NO.sub.x emissions when the fuel gas is burned. Thus, the ammonia content of the high temperature fuel gas must be decreased to reduce NO.sub.x emissions.
One way to reduce the ammonia content of the fuel gas is to promote ammonia decomposition. However, known catalysts that are active for ammonia decomposition in the range of 800-1200.degree. F. are easily poisoned by as low as a few parts per million of H.sub.2 S. At temperatures where sulfur poisoning is less of a problem (about 1400.degree. F.), these catalysts have poor mechanical/chemical stability, i.e., loss of surface area because of sintering. Similarly, catalysts that are sulfur resistant and mechanically stable at 1400.degree. F. tend not to be active enough towards ammonia decomposition at lower temperatures near 1000.degree. F. Hence, operation of an ammonia decomposition catalyst at the same temperature as a high temperature desulfurization system may not be easily implementable.
Accordingly, there is a need for a process and apparatus for reducing the ammonia concentration of high temperature fuel gas which can fit into the constraints of an IGCC power plant having high temperature sulfur removal.